Wearable devices may include sensors to collect physical data such as a heartbeat, perspiration salinity, brain waves, muscle activity, or other electrical activity. Since the electric signals collected are so minute, amplification is necessary before they can be converted from analog to digital signals and be processed digitally such as by a digital-signal processor (DSP).
Wearable devices that are small and light are desirable, but the signal quality must still be sufficient to be useful. Small physical sizes, high noise immunity, and ultra-low power consumption are design goals. Complementary metal-oxide-semiconductor (CMOS) manufacturing processes or derivates or variants are often used to produce integrated Analog Front-End (AFE) devices.
FIGS. 1A, 1B show noisy and cleaned waveforms. FIG. 1A shows an input signal that might be picked up by a sensor such as by electrodes touching a person's skin. A large amount of noise is present. FIG. 1B shows a signal output from an AFE. The noise signal of FIG. 1A has been cleaned up to produce the output signal of FIG. 1B. This signal could be a heartbeat on an electro-cardiogram (ECG or EKG) collected by the electrodes.
FIG. 2 shows an Analog Front-End (AFE). The noisy input such as shown in FIG. 1A is applied to electrodes 114, amplified, and converted to a digital signal to produce the cleaned up signal of FIG. 1B. The signal of FIG. 1B could also be the input to ADC 110 rather than its output.
Oscillator 102 generates a clock that is applied to chopping clock generator 104 to generate chopping clocks that operate choppers 116, 118 to sample signals from electrodes 114 which are buffered by chopping amplifier 12.
A Programmable-Gain Amplifier (PGA) clock is generated by PGA clock generator 106 and applied to PGA 10. PGA 10 has a gain that is programmable. Programming the gain allows the flexibility to tune the amplitude and bandwidth so that vital signs of different magnitudes and frequencies can be monitored and better isolated.
Band selection filter 112 may be a switched-capacitor filter to filter the output from PGA 10, such as to better select desired frequencies. This can be considered part of the PGA as the PGA is a 1st order low pass filter. Driver 14 drives the analog input to ADC 110, which converts this analog signal to a digital value, such as by using a Successive-Approximation Register (SAR). ADC clock generator 108 generates a clock to ADC 110.
Bias network 120 samples the input from electrodes 114 and biases chopping amplifier 12, PGA 10, and driver 14. Other biasing, control, or filters may be present.
What is desired is a Programmable-Gain Amplifier (PGA). A PGA for applications such as an AFE shown in FIG. 2 is also desired. A programmable gain amplifier using switched capacitors to program the gain is desirable. A PGA using direct charge transfer or correlated double sampling, or both, is also desired.